Contained herein is material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the United States Patent and Trademark Office patent file or records, but otherwise reserves all rights to the copyright whatsoever. The following notice applies to the software and data as described below and in the drawings hereto: Copyright(copyright) 2002, Intel Corporation, All Rights Reserved.
This invention relates to circuit boards in general, and more specifically to a circuit board with a via through a contact used for the installation of a surface mount device.
In the fabrication of printed circuit boards, multiple layers of connections may be created. In addition to other types of components used in circuit boards, certain components known as surface mount devices are mounted on the surface of circuits boards, with the mounting of a surface mount device generally being performed by soldering the leads or pins of the device to conductive pads or contacts on the surface of the circuit board. This is contrasted with, for example, components known as through hole devices with leads that are fed through holes in a circuit board, the leads then being connected to the opposite side of the circuit board.
The leads of a surface mount device may be electrically connected to other board layers by vias that are formed through the circuit board. In conventional construction, the pads for the installation of a surface mount device are connected by a trace that runs between the pad and the location for the via. Any signal or current must be transferred from the device lead through the pad, through the trace, and then through the via to the connecting layer of the circuit board.
An example of conventional construction of a circuit board is shown in FIG. 1. In FIG. 1, the circuit board 100 may contain a number of pads for the installation of one or more surface mount devices. One of the pads 105 for installation of a surface mount device is shown. Connected to the pad 105 is a trace 110 to a via 115, the via being a plated through hole created in the circuit board 100. The via 115 provides an electrically conductive path from the lead of the surface mount device to one or more other layers of the circuit board 100.
Another view of conventional construction of a circuit board is shown in FIG. 2. Please note that the dimensions of the elements shown in this figure and the other figures in this application are not necessarily drawn to scale. The figures are intended to illustrate the relationship of the elements. In particular, the thickness of the pads and traces on the circuit boards has been increased in order to provide a more clear view of the elements. In this side view, a circuit board 200 contains a number of pads for installation of one or more surface mount devices. One surface mount device pad 205 is shown in FIG. 2 on the circuit board substrate 225. Connected to the surface mount device pad 205 is a trace 210 that provides an electrical path to via 215, which is a plated through hole in circuit board 200. The via 215 is formed through the circuit board substrate 225 to produce an electrical path to one or more layers of the circuit board 200. In the conventional example shown in FIG. 2, the pad 205 is connected through trace 210 and via 215 to a connection 220. Connection 220 could be a power connection, a ground plane, or other connection to a surface mount device.
A circuit board including conventional via construction, such as that shown in FIGS. 1 and 2, suffers from certain disadvantages. The use of a trace from a surface mount device pad to the location of a via increases the length of travel of a signal or current, and thereby increases the inductance created by the electrical path loop. The increased inductance has implications in the operation of the circuit board at high speeds, with inductance having particular implications in differential signaling applications. Similarly, the longer loop increases the resistance in the electrical path, also affecting circuit board operation. Further, placement of the trace and the via on the circuit board reduces the amount of space available for other devices or connections on the board, thereby having an impact on the design of a printed circuit board and cost of board fabrication.